1. Field of the Invention
The invention relates to an apparatus for the uphill low pressure casting of metals, particularly light metals, in split sand moulds with a gate or sprue in one of the two moulds, to which is connectable the casting tube of a melt container and which is connected with the lowest area of the mould cavity, and with a sealing plug integrated into the mould and which following the filling of the mould cavity by means of an external drive engaging in the mould can be brought into the closed position.
2. Description of the Prior Art
In the uphill casting of metals the melt is placed under an overpressure in the melt container or a holding furnace, in order to force the melt via the casting tube and the sprue in rising manner into the mould cavity. Following the filling of the mould the melt column in the casting tube and in the mould sprue must be interrupted, in order to convey away the poured off mould and to be able to dock the next mould with the casting tube. Before this the melt must be returned in the casting tube. In addition, precautions must be taken with respect to the mould so that during the further conveying on of the mould the sprue is closed at least until the melt has solidified in the sprue. This takes place e.g. by moving the mould onto cooling plates or cooling plates carried along with the mould.
For sealing the mould after filling, sealing slides integrated into the mould and located at the sprue are known (WO 93/11892, WO 95/32826), which can be moved from an open position during casting into a closed position shutting off the sprue. In the case of sand moulds, either with or without boxes or flasks, the problem arises that the slide made from a different material must be segregated during the processing of the mould sand. Frequently there is an adhesion thereof to the solidified metal in the sprue and removal must take place through a separate operation.
In a known construction (WO 95/32826, DE-journal xe2x80x9cGiessereixe2x80x9d, 1998, pp 57 to 62) following the filling of the mould by means of an external drive engaging in the sand mould from the side, moulding sand is displaced from the area alongside the sprue into the latter and is intended to displace the sprue in a plug-like manner. Considerable abrasion occurs, which is displaced into the melt both in the direction of the casting tube and in the direction of the mould cavity. The moulding sand particles displaced towards the casting tube remain in the melt on return or resetting and are conveyed into the mould cavity during the next casting process, which leads to considerable quality losses with respect to the casting, particularly if it is made from light metal, e.g. aluminium alloys.
An attempt is made to prevent the displacement of moulding sand into the melt in that downstream of the sealing plug is positioned a particle filter (DE-journal xe2x80x9cGiessereixe2x80x9d), or in that (WO 95/32826), the sprue is initially guided downwards by the casting tube neck and in this falling area of the sprue the drive engages in the mould in order to displace the moulding sand in plug-like manner into the falling portion of the sprue. This construction is counter to the fundamental requirement of a very short sprue path. The falling portion is also opposed to the principle of uphill casting and can lead in this area to undesired turbulence during the pouring off of the mould. Although both constructions have the advantage that the mould contains no type-foreign components, because the sealing plug is made from the same moulding sand as the mould and consequently can be processed with the moulding sand of the mould, the disadvantage remains in both cases that sealing takes place in a completely uncontrolled manner and the quality of the seal is dependent on the shape stability, the pressing-out force acting on the moulding sand through the drive and the time force gradient. It is in particular not ensured that an intact, sealed plug is formed and that the mould does not break off laterally of the forced-out plug. These risks are particularly high with large sprue cross-sections for high casting capacities.
On the basis of this prior art, the problem solved by the invention is to provide an apparatus in which, whilst maintaining the type-pure composition of the sand mould, a completely satisfactory seal is possible in the vicinity of the sprue and also, even in the case of a high casting capacity and large sprue cross-sections, the quality of the seal remains in a reproducible manner and finally the risk of carrying moulding sand particles into the melt is minimized.
According to the invention this problem is solved in that between the sprue and the mould cavity is provided at an angle to the sprue a casting duct or channel linking the same and that the sealing plug is constructed as a sand mould body and is displaceable in a shaped-in guide of the mould between an open position and a closed position, where the sealing plug closes the casting channel and the metallostatic pressure of the melt in the mould acts perpendicular to the guide on the sealing plug.
In the construction according to the invention, the sealing plug is pre-shaped as a sand mould body. In the sand mould there is both a guide for the sealing plug and also a casting channel at an angle to the sprue, namely in the vicinity of the mould parting plane. The casting channel is preferably at right angles or under a steep, obtuse angle to the flow direction in the sprue. In the case of high castings, which consequently extend correspondingly deeply into said mould, the casting channel must also be correspondingly deep. The final cross-section of the casting channel can be determined by an inserted core.
The melt is deflected within the mould from the sprue into the casting channel. In the vicinity of the deflection is provided the guide with the sealing plug and the sealing plug is initially located in the open position freeing the transition between sprue and casting channel. Following mould filling, the sealing plug constructed as a sand mould body is displaced by means of the external drive engaging in the mould inside the guide into the closed position, in which the transition between the sprue and the casting channel is displaced away from the sealing plug. In the closed position the metallostatic pressure of the mould acts transversely to the guide on the sealing plug, so that the latter remains in its position.
As the sealing plug is a separate moulding, there is only minimum abrasion on the guide or sand mould body, so that the risk of carrying moulding sand particles into the casting tube on returning the melt is very small. It is also possible to produce the sealing plug from the same moulding sand as the mould, so that it can be processed during the preparation of the moulding sand. As a result of the separate prefabrication, the hardness of the sand mould body can be optimized in accordance with the intended use.
In a preferred development the sealing plug is held in the open position in self-locking manner in the guide. The self-locking action can be brought about by a correspondingly close fit of sealing plug and guide, optionally assisted by frictional forces due to surface roughnesses.
With horizontally positioned moulds and a roughly vertical sprue, the sealing plug can also be placed with clearance in the guide and e.g. rest on a step or the like. When filling the mould, the sealing plug is raised into the open position as a result of the melt buoyancy force and in this position it seals in the guide, e.g. engages against an annular step or is sealed against the guide surface by a conical construction of the guide. Instead of being made from moulding sand, in the manner indicated hereinbefore, the sealing plug-forming sand mould body can also be produced from core sand. In both cases a surface smoothing by the application of a blackwash is recommended.
In an other preferred development, the guide is constructed as a bush from a core sand and is inserted in prefabricated receptacles of the mould, the bush having recesses opening radially into the casting channel for melt overflow and which can be sealed by means of the sealing plug inserted in the bush.
As a result of the construction of the guide as a core sand bush it is possible to achieve better guidance characteristics for the sealing plug. The bush also has a greater strength, so that the shear forces acting on the sealing plug constitute no risk for the guide bush. For identical external dimensions and correspondingly identical receptacles in the mould, as regards arrangement and size of the recesses for the overflow of the melt, the bush can be adapted to the given circumstances. As the bush is made from core sand, it does not constitute a foreign body in the standard sand processing process.
The guide, no matter whether it is directly formed in the moulding sand or as a core sand bush, extends from the mould having the sprue, via the mould parting plane into the other mould, so that it is ensured that the casting channel positioned transversely to the sprue is completely sealable.
If the guide is formed by an inserted bush, then in both moulds are provided facing receptacles for the bush. The core sand bush has the advantage that even with a displacement of the moulds in the parting plane, a linear, smooth guidance for the sealing plug is ensured.
As a precaution, downstream of the sealing plug can be located a particle filter integrated into the mould. In the case of a separate guide bush, the particle filter can be inserted in the bush at its melt feed, so that the bush with sealing plug and particle filter can be inserted in the mould in a single operation.
With horizontal moulds, the guide for the sealing plug is equiaxial with the substantially vertical sprue and the casting channel is approximately horizontally positioned in the lower mould. Then, in the upper mould, there is a bore equiaxial with the guide for the engagement of the drive of the sealing plug from above. Thus, the drive acts in the mould from the side opposite to the sprue.
For vertical moulds with a lateral runner and an approximately horizontal sprue, the casting channel is roughly vertical and the sealing plug with its guide is positioned parallel above the sprue and transversely to the casting channel. Thus, in this case the melt initially flows horizontally to the mould and is then deflected vertically into the casting channel which, after filling the mould, is displaced by the sealing plug movable transversely thereto.
In the case of moulds with several discreet mould cavities, which are filled by means of a central sprue with radially positioned casting channels perpendicular thereto, the guide with the sealing plug is equiaxial to the sprue and the casting channels are attached to said guide, so that following the filling of all the mould cavities the casting channels can be moved with a single sealing plug.
The drive for the sealing plug is advantageously a pressure cylinder, whose piston rod engages in a shaped-in or subsequently mechanically fitted bore in the mould. The drive can preferably be controlled in such a way that a piston rod thereof, prior to the filling of the mould, is movable into a readiness position supporting the sealing plug against the pressure of the melt in the open position.